Books for a 9yr old boy *short notice urgent*

[The Poet]

I did a quick check on Amazon, and there's a ton of Heinlein available for Kindle. But take note that some of his later works are too mature for kids, as he sorta became a dirty old man at times. For instance, The Moon Is A Harsh Mistress is safe, even if the title sounds suggestive, but Stranger In A Strange Land (his masterpiece) is not suitable for a 9-year-old boy.